I'd get something that's close to what you're looking for and pay to have it modified. Especially if you've looked this long. Besides, the only thing cooler than that is if you have it made from scratch, which you could also do if you wanted. Mods are cheaper though.
I mean, there's an easy fix to short shoulder straps, and I don't think your perfect bag's gonna hit the mass market any time soon. This way you can get the look you want, as slick as you like. Besides, this way the bag's a much better story that starts off with how you looked for it for years.
Two cents, that's how I'd tackle it.